I prefer the older B20s with the 6 bolt crank. (69-73) they always seemed to be of better quality, in terms of the block itself. 74s in particular are prone to rapid and uneven cylinder wear. I had a 75 on which the top deck of the block had warped out of true. Nothing like this ever seemed to occur in my experience with a dozen or so 6 bolt engines.

The 6 bolt engines rev better too, as the rods are lighter. They are still plenty strong---never had a problem with them as long as the bearing shells were in good shape and the oil pressure was adequate.

Perhaps the best combo is a 71-73 short block (don't know exactly why, they just seem to be more durable tahn 69-70) and a 74-75 head with the biggest valves ever used as original equipment and hardened seats from the factory.